The object of the present invention is a method of compacting aerogels.
Aerogels, particularly those with a porosity above 60% and a density of less than 0.6 g/cu.cm display extremely low thermal conductivity and are therefore used as a heat insulating material, as is described in EP-A-0 171 722, as catalysts or as catalysts carriers and also as adsorption material. Furthermore, by virtue of their very low refraction index for solid substances, it is known to use them for Cerenkov detectors. Furthermore, by reason of their particular acoustic impedance, the literature describes a possible use as an impedance adaptation means, for example in the ultrasound range.
It is also possible for them to be used as carriers for effective substances in pharmacy or agriculture.
Aerogels in the broader sense, e.g. in the sense of xe2x80x9cgel with air as the dispersion agentxe2x80x9d are produced by the drying of a suitable gel. The term xe2x80x9caerogelxe2x80x9d in this sense embraces aerogels in the narrower sense, xerogels and cryogels. In this respect, a dried gel is termed an aerogel in the narrower sense when the liquid of the gel is eliminated at above critical temperature and starting from pressures above critical pressure. On the other hand, if the liquid is eliminated from the gel under sub-critical conditions, for example with the formation of a liquid/vapor interphase, then the resulting gel is frequently referred to also as a xerogel.
When the term aerogels is used in the present invention, these are aerogels in the broader sense, i.e. in the sense of xe2x80x9cgel with air as the dispersion mediumxe2x80x9d
The term does not include aerogels known from earlier literature and which are obtained for example by precipitation of silicic acid (e.g. DE 3025437, DD 296 898) or which occur as pyrogenic silicic acid, e g. Aerosil(trademark). In these cases, during manufacture, no three-dimensional gel lattice develops which is homogeneous over relatively great distances
Where aerogels are concerned, it is possible basically to differentiate between inorganic and organic aerogels.
Inorganic aerogels have already been known since 1931 (S. S. Kistler, Nature 1931, 127, 741). Since then, aerogels have been forthcoming from various starting materials. In this respect, for example SiO2-, Al2O3-, TiO2-, ZrO2-, SnO2-, Li2O-, CeO2-, V2O6-aerogels and mixtures of these were produced (H. D. Gesser, P. C. Goswami, Chem. Rev. 1989, 89, 765 el seq).
For some years, organic aerogels have also been known which are derived from the most widely diverse starting materials, e.g. melamine formaldehyde (R. W. Pekala, J. Maier, Sci. 1989, 24, 3221).
Inorganic aerogels can thereby be produced in different ways.
On the one hand, SiO2 aerogels can for example be produced by acid hydrolysis and condensation of tetra-erbyl orthosilicate in ethanol. During this process, a gel is produced which can be dried by super-critical drying while its structure is maintained. Production methods based on this drying technique are known for example from EP-A-0 396 076, WO 92/03378 or WO 95/06617
The high pressure technique involved in the super-critical drying of aerogels in however an expensive process and involves a high security risk. In addition, however, super-critical drying of aerogels is a very cost-intensive production method
An alternative to super-critical drying is afforded by a method for the sub-critical drying of SiO2 gels.
The costs involved in sub-critical drying are substantially less by reason of the simpler technology, the lower energy costs and the lesser security risk.
The SiO2 gels can for example be obtained by acid hydrolysis of tetra-alkoxy silanes in a suitable organic solvent by means of water. Once the solvent has been exchanged for a suitable organic solvent, the gel obtained is in a further step reacted with a silylating agent. The SiO2 gel resulting from this can then, from an organic solvent, be dried in air. Thus, aerogels with densities of less than 0.4 g/cu.cm and porosities above 60% can be achieved. The production method based on this drying technique is described in detail in WO 94/25149
Furthermore, the above-described gels can, prior to drying and in the alcohol-aqueous solution, be mixed with tetra-alkoxy silanes and aged, in order to increase the gel lattice strength, as disclosed in WO 92/20623.
The tetra-alkoxy silanes used as starting materials in the above-described processes do, however, likewise represent an extremely high cost factor.
A not inconsiderable cost reduction can be achieved by using water-glass as a starting material for the production of SiO2 gels To this end, it is possible for example to produce a silicic acid from an aqueous water-glass solution with the help of an ion exchange resin, the silicic acid then being polycondensed by the addition of a base to produce an SiO2 gel. After exchange of the aqueous medium for a suitable organic solvent, it is then possible in a further step to react the resulting gel with a silylating agent containing chlorine. The SiO2 gel which is surface modified for example with methyl silyl groups can then and likewise from an organic solvent, be dried in air The production method based on this technique is known from DE-A-43 42 548
Alternative methods with regard to the production of an SiO2 aerogel on a basis of water-glass with subsequent sub-critical drying are described in German Patent Application 195 41 715.1 and 195 41 992.8
Furthermore, DE-A-195 02 453 describes a use of chlorine-free silylating agents during the production of sub-critically dried aerogels.
Furthermore, an organofunctionalisation by means of organofunctionalised silylating agents in the production of sub-critically dried aerogels is described in DE-A-195 34 198.
However, on grounds of procedural technology and manufacturing costs, the production of aerogel particles on a major industrial scale is limited to particle sizes less than 5 mm and preferably less than 2 mm.
According to the particular manner of producing the aerogels, so in principle a plurality of washing and solvent exchange stages are required. Since these are diffusion-dependent, the time required is increased by the square of the radius of the gel particles. Consequently, apart from the drying method, the costs of aerogel production also increase quite considerably from the particle size upwards. On grounds of cost, the result is an endeavour to produce the smallest possible aerogel particles.
On the other hand, the handling of very small particles is very complicated and thus the costs entailed are likewise unfavourable and not every industrial application of aerogels is independent of the particle size.
Therefore, from the point of view of handling and for many applications, larger aerogel particles are needed or at least advantageous.
Therefore, the object of the present invention is to provide a method by which small aerogel particles of less than 2 mm can be formed into larger aerogel particles.